Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a thin coating of film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The photoresist coated on the substrate is next subjected to an image-wise exposure to radiation.
The radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation exposed (positive photoresist) or the unexposed areas of the photoresist (negative photoresist).
Positive working photoresists when they are exposed image-wise to radiation have those areas of the photoresist composition exposed to the radiation become more soluble to the developer solution while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working photoresist with the developer causes removal of the exposed areas of the coating and the formation of a positive image in the photoresist coating. Again, a desired portion of the underlying surface is uncovered.
Negative working photoresists when they are exposed image-wise to radiation, have those areas of the photoresist composition exposed to the radiation become insoluble to the developer solution while those areas not exposed remain relatively soluble to the developer solution. Thus, treatment of a non-exposed negative-working photoresist with the developer causes removal of the unexposed areas of the coating and the formation of a negative image in the photoresist coating. Again, a desired portion of the underlying surface is uncovered.
Photoresist resolution is defined as the smallest feature which the resist composition can transfer from the photomask to the substrate with a high degree of image edge acuity after exposure and development. In many leading edge manufacturing applications today, photoresist resolution on the order of less than 100 nm is necessary. In addition, it is almost always desirable that the developed photoresist wall profiles be near vertical relative to the substrate. Such demarcations between developed and undeveloped areas of the resist coating translate into accurate pattern transfer of the mask image onto the substrate. This becomes even more critical as the push toward miniaturization reduces the critical dimensions on the devices.
The trend towards the miniaturization of semiconductor devices has led to the use of new photoresists that are sensitive at lower and lower wavelengths of radiation and has also led to the use of sophisticated multilevel systems, such as antireflective coatings, to overcome difficulties associated with such miniaturization.
Photoresists sensitive to short wavelengths, between about 100 nm and about 300 nm, are often used where subhalfmicron geometries are required. Particularly preferred are deep uv photoresists sensitive at below 200 nm, e.g. 193 nm and 157 nm, comprising non-aromatic polymers, a photoacid generator, optionally a dissolution inhibitor, and solvent.
High resolution, chemically amplified, deep ultraviolet (100-300 nm) positive and negative tone photoresists are available for patterning images with less than quarter micron geometries.
Another recent way to improve the resolution and depth of focus of photoresists, has been to use immersion lithography to extend the resolution limits of deep uv lithography imaging. In the traditional process of dry lithography imaging, air or some other low refractive index gas, lies between the lens and the wafer plane. This abrupt change in refractive index causes rays at the edge of the lens to undergo total internal reflection and not propagate to the wafer (FIG. 1). In immersion lithography a fluid is present between the objective lens and the wafer to enable higher orders of light to participate in image formation at the wafer plane. In this manner the effective numerical aperture of the optical lens (NA) can be increased to greater than 1, where NAwet=ni sin θ, where NAwet is the numerical aperture with immersion lithography, ni is refractive index of liquid of immersion and sin θ is the angular aperture of the lens. Increasing the refractive index of the medium between the lens and the photoresist allows for greater resolution power and depth of focus. This in turn gives rise to greater process latitudes in the manufacturing of IC devices. The process of immersion lithography is described in ‘Immersion liquids for lithography in deep ultraviolet’ Switkes et al. Vol. 5040, pages 690-699, Proceedings of SPIE, and incorporated herein by reference.
For 193 nm and 248 nm and higher wavelengths immersion lithography, water is of sufficient inherent transparency so that it can be used as the immersion fluid. Alternatively, if a higher NA is desired, water's refractive index can be increased by doping with UV transparent solutes. However, for 157 nm lithography, water's high absorbance makes it unsuitable as an immersion fluid. Currently certain oligomeric fluorinated ether solvents have been used as suitable immersion fluids.
Bottom antireflective coatings are also used to prevent reflection from the various substrates used in IC processing for both dry and immersion lithography. The use of high NA lenses (typically NA greater than 1), especially in immersion lithography, with a wide range of angles of incidence, together with very diverse topographical features on substrates has reduced the effectiveness of single layer antireflective coatings. Multiple layers of bottom antireflective coatings (BARCs) with varying values of refractive index (n) and, especially, absorption (k), for each layer, provide a solution to the difficulties of dry or immersion lithography. Inorganic bottom antireflective coatings allow a gradual change in n and k values through the chemical vapor deposition (CVD) of inorganic materials, as discussed by Chen et al, in Proceedings of SPIE Vol. 4690, pg. 1085-1092, 2002. However, the process complexity of incorporating into the manufacturing process of the device another step requiring a chemical vapor deposition tool is not preferred. Similarly, combinations of organic BARCs and inorganic BARCs are not preferred, since an additional CVD tool is still required. Multiple layers of organic BARCs are more desirable since these layers are formed through a cheaper spin coating process. In most cases with smaller and more complex devices, BARCs which can form planarizing coatings are preferred. Inorganic coatings are conformal, whereas organic BARCs are capable of forming planarizing coatings, thus organic BARCs are preferred. Multiple layers of organic BARCs can provide the gradient in n and k values, but too many layers can add to the complexity of the imaging process. However a minimal number of layers, especially a two layer organic BARC stack, could provide an acceptable compromise. Thus there is a need for a simple multiple stack of organic BARCs which can effectively reduce reflection from the substrate during the imaging process of a photoresist.
The inventors of the present application have found that a process for lithography, especially immersion lithography, which comprises coating a substrate with at least two distinct organic antireflective coatings under a deep uv photoresist, where each antireflective coating having a different set of optical properties, provides unexpectedly good lithographic results.